1. Field of the Invention
The present invention relates generally to a wrist mounted wireless instrument, and more particularly to an improved antenna for a wrist mounted wireless instrument formed integral with a wrist strap for the instrument and further including a shield formed integral with the wrist strap and disposed between the antenna and the wrist to shield the antenna from interference created by the body.
2. Description of the Prior Art
Wrist mounted wireless devices, including radios, pagers, and more recently Global Positioning Satellite (GPS) receivers, are well know in the art. One of the desirable characteristics of these devices is their diminutive size and weight, and as such developers of such devices are constantly striving to reduce the size of the components used. One of the main components shared by all such devices is the antenna required for receiving, and possibly also transmitting, the electromagnetic signals of interest.
The antennas utilized by modern wrist mounted devices typically fall into either of two broad categories: antennas disposed within the case of the instrument itself, or antennas disposed in the strap that secures the instrument case to the wrist of the user. Wrist strap antennas are sometimes preferred because they do not occupy any of the already very limited space available inside the instrument case, are less susceptible to electromagnetic interference from any electric circuitry located inside the instrument case, and typically display greater sensitivity because they are not handicapped by any signal attenuation caused by the instrument case. Furthermore, when loop antennas are required, the area of the loop described by a wrist strap antenna is by necessity significantly greater than that of a case antenna.
In one of the earliest patents to address a wrist strap antenna, U.S. Pat. No. 4,769,656, Dickey discloses an antenna made of a wire mesh of individual copper conductors enclosed in an elastic fabric sleeve. The conductors are loosely woven, twisted, or braided to permit lateral expansion and also to assume a flat configuration, and are preferably woven like a coaxial cable outer shield conductor. Ganter et al. disclose in U.S. Pat. No. 4,947,179 an antenna comprised of a flexible ferrite core surrounded by a flexible coil and disposed within a wrist strap. The core may be comprised of multiple strips separated by insulating strips, thus establishing tuning capacitors.
However, wrist strap antennas do suffer from certain drawbacks. For instance, when a loop antenna having a resonance circuit is brought into contact with the wrist, the wrist provides a load relative to the resonance circuit and thus greatly reduces the quality of the circuit and the overall sensitivity of the instrument. In an attempt to solve this problem, U.S. Pat. No. 5,530,453 to Koyama discloses a loop antenna for a wrist mounted instrument that includes a resonance circuit with two capacitors having a ground therebetween, thus causing the load caused by the contact between the wrist and the antenna to be applied to only one portion of the antenna and thereby preventing deterioration of the signal received by the other portion of the antenna. Akahane et al. in U.S. Pat. No. 5,526,006 attempt to prevent the problem of wrist coupling by injection molding an antenna strip into an insulating wrist strap extending from each side of the instrument case and connecting the two strips with an electrically conductive buckle. And in yet another approach, Gehring et al. disclose in U.S. Pat. No. 5,134,724 a loop antenna having a null point at its geometric middle that coincides with the electrically conductive buckle of the wrist strap, and thereby obviates the need for insulating the buckle from the wrist.
Wrist strap antennas can also function as slot antennas, in which case they are typically formed from a conductive strip fastened to the strap and divided by a lengthwise slot across which a capacitive element is electrically connected and through which, by adjusting the capacitance value, the antenna can be tuned to a desired frequency. U.S. Pat. No. 5,589,840 to Fujisawa attempts to improve upon the gain of this type of wrist strap antenna by connecting the antenna with the instrument case so as to form a loop and further disposing the capacitive element approximately near the center of a circuit board disposed within the case such that the circuit board functions as an antenna reflector. In U.S. Pat. No. 4,922,260, Gaskill et al. disclose such an antenna comprised of multiple conductive strips forming multiple discrete capacitors, such that as the length of the wrist strap changes the tuning frequency remains the same. Kurcbart accomplishes the same goal in U.S. Pat. No. 4,977,614 by engaging a variable capacitor with a spur gear actuated by teeth on the wrist strap antenna that adjust the capacitance of the capacitor as the wrist strap slides back and forth, thereby engaging the spur gear that is mechanically connected to the variable capacitor.
The antennas described above and others like them are designed to provide acceptable performance at relatively low operating frequencies, typically below 200 MHz. As the operating frequency increases the capacitance provided by the body of the user, and more particularly the portion of the wrist encircled by the antenna, presents a parallel path of sufficiently low impedance to significantly increase the loss factor for the entire antenna.
Hence, in light of the above, those skilled in the art have recognized a need for an antenna device for a wrist mounted wireless instrument that is of reduced physical size, conformable to the shape of a typical wrist carried device such as a wrist watch, and offering sufficient sensitivity and gain to be operable at higher frequencies than have been heretofore attainable. The present invention satisfies these needs as well as others.
The present invention addresses the above mentioned needs by providing an antenna device for a wrist mounted wireless instrument comprising a strap for securing the instrument to the wrist and formed with a shield for isolating the antenna element from the effects of the body of the user.
Thus, briefly and in general terms, in one aspect the present invention is directed to an antenna device for a wrist mounted wireless instrument comprised of a wrist strap attached to the instrument case, an antenna element disposed within the strap, and an electrically conductive shield disposed within the strap between the antenna element and the wrist.
In another aspect, a wireless instrument is provided and comprises a case containing the instrument circuitry, a wrist strap connected to the case, an antenna disposed within the strap and connected to the circuitry, and an electrically conductive shield disposed in the strap between the wrist and the antenna.
In further detailed aspects the antenna device may further comprise two separate strap portions, each containing an antenna element and a shield, and may be releasably secured together by a clasp that may incorporate an electrically conductive element to electrically connect the two shields.